1. Field of the Invention
The present invention relates to elastic wave filter devices including a plurality of elastic wave resonators connected together, and particularly relates to elastic wave filter devices including serial and parallel arms.
2. Description of the Related Art
A variety of filter devices including serial and parallel arm resonators configured of surface acoustic wave resonators have been proposed. For example, WO 2011/088904 A1 and Japanese Unexamined Patent Application Publication No. 2011-101350 disclose a structure in which an acoustic velocity in an electrode finger leading end-side portion is lower than an acoustic velocity in an electrode finger intersecting portion in an IDT (interdigital transducer) electrode. It is discussed that a piston mode is formed by using the above structure so that ripples due to transverse modes can be suppressed.
Further, WO 2011/088904 A1 discloses that the width of an opening, areas in which acoustic velocities differ from each other, and so on in a serial arm resonator are set to be different from the width of an opening, areas in which acoustic velocities differ from each other, and so on in a parallel arm resonator.
Japanese Unexamined Patent Application Publication No. 2011-205625 discloses a ladder filter in which an intersecting width of a parallel arm resonator is larger than an intersecting width of a serial arm resonator. With this structure, it is expected to suppress spurious caused by transverse modes and realize the miniaturization. Making the intersecting width of the parallel arm resonator larger increases transverse mode harmonics. It is described that the increase in the transverse mode harmonics disperses the transverse mode spurious.
WO 2011/088904 A1 and Japanese Unexamined Patent Application Publication No. 2011-101350 disclose a configuration in which transverse mode ripples are suppressed by forming a piston mode. In particular, it is disclosed in WO 2011/088904 A1 that the width of the opening, shapes of the areas in which acoustic velocities differ from each other, and so on are set to be different between the serial arm resonator and the parallel arm resonator. However, the suppression of transverse mode ripples cannot be realized with certainty only by simply setting the width of the opening, the areas in which acoustic velocities differ from each other, and so on, to be different between the serial arm resonator and the parallel arm resonator.
Furthermore, it is difficult to sufficiently suppress transverse mode ripples only by simply changing the intersecting width as disclosed in Japanese Unexamined Patent Application Publication No. 2011-205625.